Some partitions have certain conventions associated with them.
Partition Convention
a Normally contains the root file system
b Normally contains swap space
c Normally the same size as the enclosing slice. This allows utilities that need to work on the entire slice (for example, a bad block scanner) to work on the c partition. You would not normally create a file system on this partition.
d Partition d used to have a special meaning associated with it, although that is now gone and d may work as any normal partition.

Are there any tutorial on newfs and bsdlabel? if that's the case, what is the command for bsdlabel that I have to use?

The FreeBSD handbooks section on adding disks (and OpenBSDs FAQ on the same issue) and the manual pages for fdisk, bsdlabel, and newfs are good. There is no need to use sysinstall, the commands and manuals are very suitable.